This invention relates to a novel and improved process for synthesizing a zeolite L type material. In particular, the preparation procedure of this invention utilizes stoichiometric, or nearly stoichiometric, quantities of raw materials such that the ratio of product to reactants approaches unity. In view of the increasing importance of developing energy efficient processing, and the need to reduce the discharge of environmentally dangerous substances, such processes have major industrial importance. Compared with conventional processes for preparing L type zeolites, the method of this invention is superior from the viewpoints of energy and raw material utilization for a given product quality and yield, and has far superior environmental characteristics.
A synthetic, crystalline potassium-containing zeolite designated as zeolite L is known to exhibit good catalytic properties, particularly for hydrocarbon conversion, and advantageous sorbent properties as described, for example, by Barrer et al., Surface Science, 12, 341 (1968). The chemical composition of zeolite L is disposed in U.S. Pat. No. 3,216,789 to be: EQU 0.9 to 1.3 (M.sub.2/n)O:Al.sub.2 O.sub.3 :5.2 to 6.9 SiO.sub.2 :xH.sub.2 O
where M is exchangeable cation of valence n and x is from 0 to 9. Zeolite L also has a characteristic X-ray diffraction pattern, and its structure has been determined by Barrer et al., Zeit. Krist., 128, 352 (1969). The X-ray diffraction pattern of zeolite L has the following more significant d (.ANG.) values:
16.1.+-.0.3 PA0 7.52.+-.0.04 PA0 6.00.+-.0.04 PA0 4.57.+-.0.04 PA0 4.35.+-.0.04 PA0 3.91.+-.0.02 PA0 3.47.+-.0.02 PA0 3.28.+-.0.02 PA0 3.17.+-.0.01 PA0 3.07.+-.0.01 PA0 2.91.+-.0.01 PA0 2.65.+-.0.01 PA0 2.46.+-.0.01 PA0 2.42.+-.0.01 PA0 2.19.+-.0.01 PA0 7K.sub.2 O:Al.sub.2 O.sub.3 :20SiO.sub.2 :450H.sub.2 O; and PA0 8K.sub.2 O:Al.sub.2 O.sub.3 :10SiO.sub.2 :500H.sub.2 O. PA0 0.9.+-.0.2R.sub.2/w O:Al.sub.2 O.sub.3 :5.0.+-.1.5SiO.sub.2 :wH.sub.2 O PA0 SiO.sub.2 /Al.sub.2 O.sub.3 of from 6 to 30, PA0 R.sub.2/v O/SiO.sub.2 of from 0.30 to 0.70, and PA0 H.sub.2 O/R.sub.2/v O of from 80 to 140; PA0 1.05.+-.0.3M.sub.2 O:Al.sub.2 O.sub.3 :4.0-7.5SiO.sub.2 PA0 1.0 to 1.1M.sub.2 O:Al.sub.2 O.sub.3 :5.0 to 7SiO.sub.2 :1 to 6H.sub.2 O PA0 1.0 to 1.1M.sub.2 O:Al.sub.2 O.sub.3 :5.0 to 7SiO.sub.2 :1 to 6H.sub.2 O PA0 0.9 to 1.3(Na,K).sub.2 O:Al.sub.2 O.sub.3 :5.2 to 6.9SiO.sub.2 0 to 9H.sub.2 O
The preparation of zeolite L described in U.S. Pat. No. 3,216,789 involves crystallizing the zeolite from a reaction mixture having a mole ratio of silica to alumina which is significantly higher than the ratio in the formed zeolite. Specifically, the reaction mixture comprises mole ratios:
______________________________________ K.sub.2 O/(K.sub.2 O + Na.sub.2 O) 0.33-1 (K.sub.2 O + Na.sub.2 O)/SiO.sub.2 0.35-0.5 SiO.sub.2 /Al.sub.2 O.sub.3 10-28 H.sub.2 O/(K.sub.2 O + Na.sub.2 O) 15-41 ______________________________________
Another typical preparation of zeolite L as disclosed by Breck, Zeolite Molecular Sieves, New York: J. Wiley, 283 (1974) employs an excess of SiO.sub.2 and a greater excess of K.sub.2 O. Typically the excess KOH in the effluent must be neutralized with sulphuric acid, and the excess silica must be precipitated, filtered, and either reused or discarded. The final effluent will still contain colloidal silica which must be removed by polishing processes before discharge. Recycle processes using the mother liquor are usually rapidly degraded because impurity nuclei are also recycled and these rapidly overwhelm the primary product, resulting in the discard of whole batches. In the K,NaL or KL synthesis system, common impurities showing this behavior are phillipsite and merlinoite (Passaglia et al., Amer. Mineralogist, (1978), p. 355), also called KM (Barrer et al., J. Chem. Soc. (1956) p. 2882) or Linde W (U.S. Pat. No. 3,012,853).
British Pat. No. 1,202,511 describes a revised zeolite L preparation using lower proportions of silica in the reaction mixture which comprises mole ratios of reactants as follows:
______________________________________ K.sub.2 O/(K.sub.2 O + Na.sub.2 O) 0.7-1 (K.sub.2 O + Na.sub.2 O)/SiO.sub.2 0.23-0.35 SiO.sub.2 /Al.sub.2 O.sub.3 6.7-9.5 H.sub.2 O/(K.sub.2 O + Na.sub.2 O) 10.5-50 ______________________________________
The ratio H.sub.2 O/(K.sub.2 O+Na.sub.2 O+SiO.sub.2 +Al.sub.2 O.sub.3) is preferably not greater than 6 to give a "dry gel" as product. The source of silica employed is a solid, amorphous silica.
U.S. Pat. No. 3,867,512 discloses a preparation of zeolite L from a reaction mixture having a molar composition:
______________________________________ K.sub.2 O/(K.sub.2 O + Na.sub.2 O) 0.3-1 (K.sub.2 O + Na.sub.2 O)/SiO.sub.2 0.3-0.6 SiO.sub.2 /Al.sub.2 O.sub.3 10-40 H.sub.2 O/(K.sub.2 O + Na.sub.2 O) 15-140 ______________________________________
in which the silica source is a gel having at least 4.5 weight percent water and prepared in a particular manner.
L. Wilkosz in Pr Chem 409 (1974)--Chemical Abstracts, 90 (1979) 573478 describes the preparation of zeolite L from a synthesis sol prepared by treating a solution containing silica, potassium hydroxide and sodium hydroxide with a second solution containing potassium aluminate, potassium hydroxide and sodium hydroxide and crystallizing for 72 hours at 20.degree. C. and 122 hours at 100.degree. C. The zeolite L product has a SiO.sub.2 :Al.sub.2 O.sub.3 ratio of 6.4:1, derived from input stoichiometries having SiO.sub.2 /Al.sub.2 O.sub.3 ratios between 15 and 30.
G. V. Tsitsishvilli et al. in Doklady Akademii NaikSSSR, 243, 438-440 (1978) describe the synthesis of zeolite L from alumina-silica gels containing tributylamine. The gels used had the following molar ratios:
______________________________________ SiO.sub.2 :Al.sub.2 O.sub.3 25 (K.sub.2 O + Na.sub.2 O):Al.sub.2 O.sub.3 18 (K.sub.2 O + Na.sub.2 O):SiO.sub.2 0.72 H.sub.2 O/(K.sub.2 O + Na.sub.2 O) 20 K.sub.2 O:Na.sub.2 O 0.5 ______________________________________
Y. Nishiimura in Nippon Kagaku Zasshi, 91, 1046-9 (1970) describes in general terms zeolite L preparation from a synthesis mixture containing colloidal silica, potassium aluminate and potassium hydroxide having a SiO.sub.2 :Al.sub.2 O.sub.3 ratio of 15-25, but exemplifies only two synthesis mixtures having the following ratios of components:
Other workers have prepared zeolite L from gels, mixed base systems and metakaolin. See, e.g., Aiello and Barrer, J. Chem. Soc. Dalton, 1470 (1970); Barrer et al., J. Chem. Soc. Dalton, 1258 (1972) ; Barrer et al., J. Chem. Soc. Dalton, 934 (1974); and U.S. Pat. No. 3,867,512 to Young. All of these syntheses, however, employ large excesses of reactants and do not result in stoichiometric conversions.
U.S. Pat. No. 3,298,780 describes zeolite UJ having a composition, expressed as mole ratios of oxides, corresponding to
wherein R represents at least one cation having a valence of not more than 4, v represents the valence of R and w can be any value up to about 5, said zeolite having an X-ray powder diffraction pattern essentially as shown in the following table:
______________________________________ Interplanner spacing, d (.ANG.) Relative Intensity ______________________________________ 16.25 .+-. 0.25 VS 7.55 .+-. 0.15 M 6.50 .+-. 0.10 M 5.91 .+-. 0.10 W 4.61 .+-. 0.05 S 3.93 .+-. 0.05 S 3.67 .+-. 0.05 W 3.49 .+-. 0.05 M 3.29 .+-. 0.05 W 3.19 .+-. 0.05 M 3.07 .+-. 0.05 M 2.92 .+-. 0.05 M 2.66 .+-. 0.05 W ______________________________________
prepared by a process comprising preparing an aqueous reactant solution having a composition, expressed as mole ratios of oxides, corresponding to
and maintaining said reactant solution at a temperature between 150.degree. F. (65.6.degree. C.) and 325.degree. F. (162.8.degree. C.) until the zeolite crystals are formed Zeolite UJ is described as having nearly cubic shaped crystals with a crystal size ranging upward from 0.05 micron.
GB. No. 1,393,365 describes zeolite AGl, related to zeolite L, having the molar composition other than water:
wherein M is potassium or a mixture of potassium and sodium, a characteristic X-ray powder diffraction pattern, and being capable of adsorbing at least 3% w/w perfluorotributylamine. As the zeolite L pore structure is too small to allow penetration by this molecule, zeolite AG-1 cannot be highly pure zeolite L.